An important aspect of AMTI radar systems performance involves their capability of detecting airborne targets against high clutter backgrounds and in severe jamming environments; and this capability is a direct function of their antenna mainlobe-to-sidelobe response ratio. Since targets in the main beam can have extremely small radar cross sections, they must compete with clutter returns both in the main beam and in those side lobes which might occur in the same range cell as the target. With high or medium pulse repetition frequency (PRF) doppler radar mechanizations, close-in, strong sidelobe clutter returns can totally obscure targets at longer ranges because of their inherent range ambiguity. In low PRF systems, where range is unambiguous, the target in the main beam needs only to compete with sidelobe clutter which occurs at the same range. However, strong fixed sidelobe clutter discrete signals at other ranges are frequently detected and will be treated as bona fide moving targets because of the AMTI/time averaged clutter coherent airborne radar (TACCAR) mechanization typically employed in such radars for minimizing beam clutter spread and thus, they will be tracked until their lack of target validity is established. This is a significant problem in such radars due to these sidelobe discrete signals overloading the track processor.
Incidentally, TACCAR mechanizations are well known, and are discussed for example in the following text, entitled "Radar Handbook" by Merrill Skolnik, McGraw Hill, 1970, in the chapter entitled "MTI Radar-IF Cancellers", see pp. 17-32 to 17-37, and the bibliography on page 17-60, in which attention is particularly directed to footnote No. 15, the article referenced on page 17-32.
Briefly, TACCAR, or Time Averaged Clutter Coherent Airborne Radar, involves clutter rejection in the main beam of the antenna, through a clutter referencing process, which locks the main beam clutter to the pulse repetition frequency (PRF) lines of the radar transmitting waveform. This is accomplished by driving the coherent or "stable" local oscillator (STALO) so as to always maintain the ground clutter spread around the pulse repetition lines (representing zero doppler shift). However, this has the effect of producing an artificially induced doppler shift in returns on the side lobes from stationary targets, and they will now appear as moving targets in the radar processor and will be tracked as bona fide targets. They will fluctuate tremendously and eventually will drop out of the tracker as false targets, but meanwhile they can create a serious computer overload situation. Incidentally, the use of the term and eventually will "coherent" with respect to the local oscillator is to be preferred as compared with the use of the term "stable", as its frequency is varied slightly, by the TACCAR process.
Of course, in the absence of the use of the STALO/TACCAR process, the returns from stationary side lobe objects or clutter would be rejected by the normal moving target identification (MTI) process which cancels out returns which do not shift in position during successive scans, such as in ground-based MTI radars.
Accordingly, one principal object of the present invention is to identify and to reject those STALO/TACCAR induced false targets which arise from the sidelobe returns from fixed or stationary objects, in airborne MTI radars, and also, to apply the basic Sidelobe Discrimination technique to other types of radars in which rejection of any sidelobe clutter is important, including ground-based systems.
It is noted in passing that sidelobe discrimination methods have previously been proposed, in conventional directional antenna receiving systems, see U.S. Pat. No. 4,266,226, granted May 5, 1981, by way of example. However, the system disclosed in this patent does not address the radar system's problems. Normally, in conventional radar systems which do not use TACCAR/STALO mechanizations, the returns from fixed objects picked up by the sidelobes, could be eliminated by conventional moving target identification techniques, provided they are not of large amplitudes. In accordance with the present invention, radar systems utilizing TACCAR or similar processes are faced with the unusual situation that sidelobe fixed target returns are converted into spurious apparently moving targets; and the present invention involves the elimination of this problem.